The present invention relates to alpha-aminophosphonic acids, and derivatives thereof, and methods of preparing them. More specifically, the invention pertains to synthetic routes in which the chirality of an amino acid starting material is preserved during a series of reactions to produce an alpha-aminophosphonic acid or alpha-aminophosphonate.
Aminophosphonic acids and aminophosphonates are derivatives of amino acids in which the amino acid carboxyl group has been replaced with a phosphonic acid or phosphonate moiety. In alpha-aminophosphonic acids and phosphonates, the alpha-carbon atom is often a chiral center, bound to a phosphonate moiety, an amine moiety, and one or more amino acid side chains. The structure can be represented as follows: ##STR1## in the displayed structure, R represents any amino acid side chain and R.sup.3 and R.sup.4 represent hydrogen (in the case of a phosphonic acid) or a group such as alkyl or aryl (in the case of a phosphonate ester).
Alpha-aminophosphonates have various uses; many of which involve synthesis of peptide analogs (peptidylphosphonates) that possess a phosphonate linkage in the place of at least one amide link in the peptide main chain. Because the phosphonate linkage exists as a charged moiety in the peptide backbone, it increases the water solubility of the peptide. Further, the phosphonate linkage can impart protease resistance and therefore increase the serum half-life of many therapeutic peptides.
In addition, substitution of the phosphonate linkage for the amide linkage allows for the introduction of additional functionalities into regions of space inaccessible in naturally occurring peptides. Specifically, amide linkages are planar, so peptides have a flat configuration in the area of the carboxylic acid linkage. Phosphonate linkages, however, are tetrahedral. This tetrahedral geometry allows for the presence of substituents in the areas above and below the plane of the carboxylic acid linkage of a peptide. Moreover, the tetrahedral configuration of the phosphonate linkage can be exploited to optimize enzyme-inhibitor or ligand-receptor binding. See Bartlett and Marlowe (1983) Biochemistry 22:4618-24.
Literature examples of specific uses of peptidylphosphonates are numerous; these compounds are recognized as effective transition-state analog inhibitors for a variety of enzymes, including a number of proteases and esterases (see, e.g., Morgan et al., (1991) J. Am. Chem. Soc. 113:297 and Bartlett et al.,(1990) J. Org. Chem. 55:6268). Peptidylphosphonate esters have been used as nonhydrolyzable analogs of phosphates to inhibit dinucleoside triphosphate hydrolase (see, e.g., Blackburn et al., (1987) Nucl. Acids Res. 15:6991), phosphatidyltransferase (see, e.g., Vargas et al. (1984) Biochim. Biophys. Acta 796:123), and squalene synthetase (see, e.g, Biller et al. (1988) J. Med. Chem. 31:1869). The most potent noncovalent enzyme inhibitor known is a phosphonyltripeptide inhibitor of carboxypeptidase A, which binds with 11 femptomolar (fM) K.sub.d (see Kaplan et al. (1991) Biochem. 30:8165-8170). In addition, phosphonate esters have been used as haptens for the production of catalytic: antibodies possessing esterase activity (see, e.g., Jacobs et al. (1987) J. Am. Chem. Soc. 109:2174; Tramontano et al. ( 1986) Science 234:1566; and Pollack et al. (1986) Science 234:1570; see also, U.S. patent application Ser. No. 07/858,298, filed Mar. 26, 1992). Some peptidylphosphonate analogs are commercially available. For instance, the drugs Monopril and Fosinopril are available from Bristol Myers, Squibb (Evansville, Ind.).
Recently, innovative combinatorial strategies for synthesizing large numbers of polymeric compounds on solid supports have been developed. One such method, referred to as VLSIPS.TM. ("Very Large Scale Immobilized Polymer Synthesis"), is described in U.S. patent application Ser. No. 07/805,727, filed Dec. 6, 1991, which is a continuation-in-part of Ser. No. 07/624,120, filed Dec. 6, 1990, which is a continuation-in-part of U.S. Pat. No. 5,143,854, which is a continuation-in-part of Ser. No. 07/362,901, filed Jun. 7, 1989, and now abandoned. Such techniques are also described in PCT publication No. 92/10092. Related combinatorial techniques for synthesizing polymers on solid supports are discussed in U.S. patent application Ser. No. 07/946,239 filed Sep. 16, 1992 which is a continuation-in-part of Ser. No. 07/762,522 filed Sep. 18, 1991 and U.S. patent application Ser. No. 07/980,523 filed Nov. 20, 1992 which is a continuation-in-part of Ser. No. 07/796,243 filed Nov. 22, 1991. Briefly, a combinatorial synthesis strategy is an ordered strategy for parallel synthesis of diverse polymer sequences by sequential addition of reagents to a solid support. These techniques can be extended to produce immobilized peptidylphosphonates in large arrays of polymers. For example, U.S. patent application Ser. No. 07/943,805 discusses the use of monoesters of alpha-aminophosphonic acids to synthesize peptidylphosphonates on solid supports. Each of the references mentioned in this paragraph are incorporated herein by reference for all purposes.
Methods of preparing amino phosphonates are known. For example, they can be prepared by the addition of phosphite to imines. In addition, they may be produced from amino acids by the procedure of Corcoran et al. (1990) Tetr. Lett. 31:6827-6830. The Corcoran et al. reference describes the oxidative decarboxylation of N-protected amino acids with lead tetraacetate to form the corresponding O-acetyl-N,O-acetal. Subsequent treatment with a phosphite yields the corresponding phosphonate or phosphonic acid. Further, compounds having the following structure where Z is either iodo, bromo, or chloro, can be prepared through the treatment of an N-protected amino acid with lead tetraacetate and the appropriate halide ions. ##STR2##
This general procedure is reported in Kochi (1965) J. Am. Chem. Soc. 87:2500, and a review of this reaction can be found in Sheldon and Kochi (1972) Org. React. 19:279-421. Alternatively, the above compound, where Z is either iodo, bromo, or chloro, can be prepared from an N-protected amino acid using the procedures outlined in March, 1985, Advanced Organic Chemistry 3rd Ed., (John Wiley & Sons, New York), pp. 654-655.
The corresponding alpha-aminophosphonate can be prepared through the reaction of the above compound, where Z is acetoxy or halo, with a suitable phosphite such as trimethylphosphite. See Corcoran, supra; see also, Seebach et al. (1989) Helv. Chim. Acta 72:401. A review of this general reaction can be found in Arbuzow (1964) Pure Appl. Chem. 9:307. For example, when Z is acetoxy, the above acyl compound may be treated with trimethylphosphite in the presence of titanium tetrachloride to produce the trimethyl ester of an alpha-aminophosphonic acid.
However, the above procedure will not produce a stereospecific alpha-amino phosphonic acid. As the treatment of the acetoxy compound with TiCl.sub.4 generates an iminium ion which destroys any chirality that may have been present in the starting materials, the decarboxylation step (as employed in the procedure of Corcoran et al.) does not result in any significant enantiomeric excess. Moreover, even if racemic aminophosphonic acids are acceptable, the harsh reaction conditions employed in the above synthesis can often limit applicability to simple amino acids.
Various other syntheses of aminophosphonic acids have been reported. For example, the side chain R group can be attached to a suitably protected aminomethylphosphonic acid. However, the available methods are limited in that either they produce racemic alpha-aminophosphonic acids and/or they work with only a few amino acids.
Shiozaki (1990) Synthesis 691-693, which is incorporated herein by reference for all purposes, describes a synthetic route for decarboxylating alpha-amino acids with retention of configuration. In this method, amine protected alpha-amino acids are coupled to 3-chloroperoxybenzoic acid or peroxybenzoic acid in the presence of 1,3-dicyclohexylcarbodiimide (DCC) to give the corresponding acyl aroyl peroxide shown below. ##STR3## AA is an amino acid residue (i.e. protected amine group, alpha carbon, and side chain) and Ar is an aryl group from the peracid (peroxybenzoic acid or 3-chloroperoxybenzoic acid in Shiozaki). For some alpha-amino acids, most notably those in which the protected alpha-amine group contains no hydrogen atoms, the acyl aroyl peroxide undergoes oxidative decarboxylation to give an ester (shown below) with retention of configuration (see Shiozaki, supra). ##STR4## The Shiozaki article shows how this reaction might be used in carbapenem synthesis. The mechanism of the rearrangement to the acetoxy compound is described in Denney and Sherman (1965) J. Org. Chem. 30: 3760.
The method of Shiozaki might be a potentially useful decarboxylation step in a synthetic scheme for preparing alpha-aminophosphonic acids with retention of configuration. However, a suitable synthesis must include other steps that ensure that the configuration is preserved throughout the entire synthesis. Thus, for example, the use of TiCl.sub.4 with a phosphite as employed in the method of Corcoran et al. would be unacceptable. It is apparent that there remains a need for a mild stereospecific method of preparing alpha-aminophosphonates from corresponding amino acids.